It depends on what you want to do. There are several options.
If you're just wanting to crop the screen, then you can hold alt and drag the boundary of the screen capture to the desired position. You can also perform the same action by setting the crop boundaries within the Edit Transform menu.
If you want to cover up that portion of the screen, then just create a new source (either with an image, video, or whatever you want), and you can also set a hotkey to show/hide the source as needed.
If you want to just entirely cut out a portion of the screen, then you can use an image mask. Create an image (.png preferred, but anything works) that has the same resolution as your capture, paint everything that should be shown white, and paint the section that should be hidden as black. Save that image, then add an image mask filter to your capture source using that image.
There's plenty of other options as well, including all sorts of different things you could do with layer groupings/scene sources and filters on top of that, but that's all dependent on what exactly you're looking for.
